1. Field of the Invention
The present invention relates, in general, to controllers for cross-point switches, and, more particularly, to methods and structures for high speed connection set up in a fibre channel switch.
2. Relevant Background
Fibre Channel is a high performance serial interconnect standard designed for bi-directional, point-to-point communications between servers, storage systems, workstations, switches, and hubs. It offers a variety of benefits over other link-level protocols, including efficiency and high performance, scalability, simplicity, ease of use and installation, and support for popular high level protocols.
Fibre channel employs a topology known as a "fabric" to establish connections between ports. A fabric is a network of switches for interconnecting a plurality of devices without restriction as to the manner in which the switch can be arranged. A fabric can include a mixture of point-to-point and arbitrated loop topologies.
In Fibre Channel a channel is established between two nodes where the channel's primary task is to transport data from one point to another at high speed with low latency, performing only simple error detection in hardware. The Fibre channel switch provides flexible circuit/packet switched topology by establishing multiple simultaneous point-to-point connections. Because these connections are managed by the switches or "fabric elements" rather than the connected end devices or "nodes", fabric traffic management is greatly simplified from the perspective of the device.
To connect to a fibre channel fabric devices include a node port or "N.sub.-- Port" that manages the fabric connection. The N.sub.-- port establishes a connection to a fabric element (e.g., a switch) having a fabric port or "F.sub.-- port". Devices attached to the fabric require only enough intelligence to manage the connection between the N.sub.-- Port and the F.sub.-- Port. Fabric elements include the intelligence to handle routing, error detection and recovery, and similar management functions.
A switch is a multi-port device where each port manages a simple point-to-point connection between itself and its attached system. Each port can be attached to a server, peripheral, I/O subsystem, bridge, hub, router, or even another switch. A switch receives a connection request from one port and automatically establishes a connection to another port. Multiple calls or data transfers happen concurrently through the multi-port Fibre Channel switch. A key advantage of switched technology is that it is "non-blocking" in that once a connection is established through the switch, the bandwidth provided by that connection is not shared.
Data connections in a fibre channel fabric tend to be short lived, hence, a switch that takes a long time to set up connections through a switch will severely limit the effective bandwidth of the switch. It typically requires a number of machine clock cycles to set up the connection after the connection request is made. From the time that a data connection is requested at one port until that connection is actually set up, no data can flow between the two affected ports.
In prior approaches, a multi-port switch processed connection requests from the various ports sequentially such that if one connection request is issued other subsequent connection requests would be blocked until the first connection request was processed. This approach used a common control bus shared by a plurality of ports to transport connection requests between the ports and the crossbar control logic. Because of this, a single failure could affect multiple ports. Also, connection setup was handled by a single pipeline that handled the connection requests for each of the ports. With up to 68 ports per switch (or more), latency caused by bus contention during the connection setup phase has become a significant bottleneck in overall throughput of the switch.
Another limitation of prior systems was that a port issuing a connection request had no knowledge about whether the destination port that it was attempting to reach was busy. Hence, the requesting port had to proceed through the entire connection request process, which could take multiple clock cycles, only to determine that the connection failed. However, the busy state of the destination port may have been set before the connection request process had begun. A need exists for communicating busy status of a port back to a port requesting a connection to the busy port so that the requesting port can abort or delay its request so as to lessen consumption of switch resources.